The present invention relates to an ultrasonic diagnostic apparatus and, more particularly, to an ultrasonic diagnostic apparatus comprising a plurality of ultrasonic transducers having different focal lengths.
In general, in an ultrasonic diagnostic apparatus, an ultrasonic transducer such as a piezoelectric element is positioned close to an object to be examined. Then, a high-frequency AC voltage of a MHz-band corresponding to a resonance frequency is applied to the transducer for an extremely short time period so that the transducer is resonated, thereby causing the transducer to emit ultrasonic pulses. In this case, if the object to be examined is a uniform medium, the ultrasonic pulses propagate straight through the medium. However, if a boundary is present between tissues having different acoustic impedances, some pulses are reflected but others are transmitted. These reflected echos are received by the transducer, and a distance between the transducer and the boundary is measured in accordance with a speed of the ultrasonic wave and a time required for the ultrasonic pulse to reciprocate. When echo signals obtained from one transmitted pulse are aligned in time sequence, an image signal representing slice information of the tissue along the transmission direction is obtained. In a radial scanning type diagnostic apparatus, the transducer is rotated in a slice of the object to be examined. Therefore, by causing the transducer to transmit ultrasonic pulses n times during each rotation, an image signal in a scanning line is obtained by dividing a circle at n equal angular intervals, thereby obtaining a tomographic image of the object to be examined.
In this case, a resonance frequency of the transducer is predetermined, and as the resonance frequency is increased, resolution is increased. However, attenuation of the ultrasonic wave is also increased, thereby shortening a distance through which the ultrasonic wave propagates. On the contrary, if the resonance frequency is low, attenuation is small, so that the ultrasonic wave can propagate over a long distance. However, since resolution is poor, diagnosis of a detailed portion cannot be performed. For this reason, in order to obtain a clear image in a wide range from short to long distances, a total distance is divided into a plurality of short distances, and a transducer having an optimal resonance frequency is assigned to each short distance, i.e., a plurality of transducers must be provided.
Such a conventional ultrasonic diagnostic apparatus is disclosed in Japanese Patent Disclosure (Kokai) No. 61-11026. This apparatus comprises a probe having at its distal end a flexible tube to be freely inserted/removed with respect to a body cavity, and two ultrasonic transducers having different resonance frequencies and incorporated in the distal end of the flexible tube. An ultrasonic lens having a short focal length is mounted on the ultrasonic transducer having a high resonance frequency, and an ultrasonic lens having a long focal length is mounted on the ultrasonic transducer having a low resonance frequency. In this apparatus, the two transducers are simultaneously driven to obtain close and remote images. Then, the close and remote images are synthesized into a single image, and this single image is displayed in real time.
However, in this conventional apparatus, the transducers having different resonance frequencies are used. Therefore, drivers having different frequencies must be provided, so that a circuit arrangement of the driver is undesirably complicated.
This problem is not limited to an intracorporeal ultrasonic diagnostic apparatus but can be applied to an extracorporeal diagnostic apparatus in which a probe is in contact with the body surface and which probe comprises transducers having different focal lengths and different resonance frequencies.